1. Field of the Invention
The present invention generally relates to arrhythmia classification, and in particular to an implantable medical device capable of detecting and classifying an arrhythmia and to an arrhythmia classification method.
2. Description of the Prior Art
Implantable medical devices (IMDs), including implantable cardioverter-defibrillators (ICDs) and pacemakers, can today be used for detecting and combating arrhythmias, such as ventricular tachyarrhythmia in IMD patients. Ventricular tachyarrhythmias, for example ventricular fibrillation or tachycardia, need to be detected as early as possible as they may otherwise lead to the death of the patient if not quickly terminated. As a consequence, once tachyarrhythmia is detected, the IMD will combat it by delivering one or more defibrillation or cardioversion shocks.
However, in several IMD and ICD patients about 5-15% of the defibrillation or cardioversion shocks are in fact unnecessary. This means that worst-case approximately one shock out of five is inappropriate for these patients. Every inappropriate shock causes decreased quality of life for patients, battery depletion of the device and potential induction of dangerous arrhythmias. Unnecessary shocks should therefore be minimized.
U.S. Pat. No. 5,311,874 discloses a method for tachycardia discrimination. In a first embodiment, a cardiac biopotential signal is recorded and processed to identify a number of feature values representing maximum and minimum values of a complex in the signal, corresponding to a complete cardiac cycle. Firstly, the complex is classified as a baseline complex or a non-baseline complex based on the cycle length of the complex. If the complex is classified as a non-baseline complex, an extensive and very complex processing of its feature values is conducted to subsequently arrive at a discrimination point in a plane defined by a similarity vector and a dissimilarity vector. Depending on where this point is in the plane, the non-baseline complex is classified as a ventricular tachycardia (VT) or non-VT complex. In a second embodiment, a corresponding complex signal processing is performed but for discriminating between hemodynamically stable and unstable ventricular tachycardias. In this case, the input signal can be a signal or condition related to the hemodynamics of the heart, such as pressure, flow or impedance.
US 2005/0154421 discloses a technique for reducing inappropriate delivery of therapy to treat ventricular tachyarrhythmias caused by supraventricular tachycardia (SVT). The document specifies that SVT can be conducted to the ventricles and lead to short ventricular cycle lengths (VCLs) that would imply ventricular tachyarrhythmia. Their technique is based on measuring multiple VCLs over a defined time period. The number of such cycles that have a length shorter than a given threshold is determined and used as a basis for detecting ventricular tachyarrhythmia. If tachyarrhythmia is detected, it is determined whether the tachyarrhythmia is due to SVT or may indeed be lethal. This determination can be based on measured VCLs and atrial cycle lengths (ACLs), measured activity level of the patient or intracardiac pressure measurements.
US 2007/0043394 discloses an IMD having circuitry for sensing cardiac signals and determining an intracardiac impedance signal. Cardiac cycles of a subject are determined based on the sensed cardiac signal and tachyarrhythmia is detected using cardiac cycle to cardiac cycle changes in a plurality of intracardiac impedance parameters obtained from the intracardiac impedance signal.
There is still a need for a technique and IMD capable of accurately classifying arrhythmias in order to select appropriate treatments or select not to apply any combating treatment to the subject.